Battery systems that include battery cells connected in series can output high voltage. Furthermore, battery systems that include battery cells connected in parallel can secure high charge-discharge current.
For example, in high-current and high-output battery systems that have been used as power supplies for motors driving vehicles, multiple battery cells are connected in series in order to secure high voltage output. In the batteries systems used for that purpose, the multiple battery cells are connected with bus bars of metal plates. The bus bars are connected to electrode terminals of the battery cells, which form the battery systems, by way of laser welding. To form such connection structures, cut parts are formed in the bas bars, the electrode terminals of the battery cells are inserted into the cut parts, boundary regions between the inserted electrode terminals and the bus bars are irradiated with laser beams, and thus, the electrode terminals and the bus bars are joined to one another based on melting of the boundary regions.
Positive electrodes and negative electrodes are provided in the batteries, and, conventionally, aluminum terminals have been used on the positive-electrode sides, while nickel-plated copper terminals have been used on the negative-electrode sides. The electrodes of adjacent battery cells are inserted into the respective cut parts of the bus bars, and thus, the adjacent battery cells are connected with each other in series or in parallel. That is, electrode terminals of at least two battery cells are connected to one bus bar.
When combined components of aluminum and copper that are called as clad materials are used for the bus bars, the aluminum terminals on the positive-electrode sides should be welded onto the aluminum sides of the clad materials, and the copper terminals on the negative-electrode sides should be welded onto the copper sides of the clad materials. Since homogenous metal materials are welded onto one another in this case, there are not any particular technical difficulties.
However, the clad materials are formed by stacking thin aluminum plates and thin copper plates in such a manner that their joint parts come into contact with each other, followed by joining the aluminum and copper plates together based on pressure bonding simultaneously with application of heat thereto. Therefore, costs required for the step will be high, and also, materials required therefor will be expensive. Thus, there is a problem that it is impossible to reduce the costs.
In order to cope with such a situation, by using aluminum, which is an inexpensive material, for bus bars, it becomes possible to produce inexpensive and lightweight battery systems. However, when aluminum bus bars are employed, it would be very difficult to stably realize high-quality welding on the negative-electrode sides since heterogeneous materials, i.e., aluminum bus bars and nickel-plated copper terminals, need to be welded, although there would be no problem on the positive-electrode sides since homogeneous materials, i.e., aluminum bus bars and aluminum terminals, need to be welded.
In such a heterogeneous-material welding process, different types of metal materials are caused to melt and mix together, and then, are caused to solidify to thereby complete the welding process. However, in cases of heterogenous-material welding of aluminum and copper, intermetallic compounds including aluminum elements and copper elements at constant ratios will be formed if their alloys are sufficiently heated, and are fused for a certain period of time above a certain temperature. The intermetallic compounds refer to compounds (alloys) that are formed by two or more types of metals. Atom ratios of the constituent elements are expressed by integers. The compounds often have distinctive physical/chemical properties, and crystal structures that are different from those of the constituent elements.
Hereinafter, intermetallic compounds will be described in brief. Intermetallic compounds are compound formed of two or more types of metals, and are one type of alloy. Alloys are generally classified into solid solutions and intermetallic compounds.
Solid solutions are alloys in which two or more elements (that may be metal or non-metal elements) are combined with one another, thereby forming a homogenous solid phase as a whole. In solid solutions, while a structure of one of the metal elements is retained, atoms of the one of the metal are randomly substituted with the other metal element(s), or the other metal element(s) penetrates into the structure. Their compositions vary within a certain range. The properties would be similar to those of the original metals. In cases of solid solutions of aluminum and copper, the solid solutions would be softer, and have ductibility, compared with their intermetallic compounds.
On the other hand, intermetallic compounds have crystal structures that are completely different from those of the original metals. As a result, a given number of metal atoms are located at given positions, and thus, intermetallic compounds have compositions with simple integer ratios. Their properties are different from those of the original metals. In cases of intermetallic compounds of aluminum and copper, there are mainly CuAl2, CuAl, and Cu9Al4, and they have hard and fragile properties, compared with solid solutions.
If, in heterogenous-material welding processes in which aluminum and copper materials are stacked and in which laser beams are applied onto aluminum sides of the stacked materials, large amounts of intermetallic compounds are produced in the joint areas, a small degree of detachment would be caused in boundary faces of hard and fragile intermetallic compounds when stress due to tension or the like is caused. As a result, such detachment would be broadened, and thus, the entire joint face would be stripped off even due to small stress. Therefore, there has been a problem that the above heterogenous-material welding process cannot be employed for joining components that will be subjected to stress.
With regard to a technique for improving joint strength in butt heterogenous-material welding, a joint material, including: a first metal component including Al, and less than 5.7 wt % of Cu; a second metal component including Cu, and less than 9.4 wt % of Al; and a joint part that joins the first metal component and the second metal component, wherein the joint part includes at least one element selected from the group consisting of Si, Ni, Mn, Co, Zn, Ge, Au, Ag and Pd has been disclosed in JP-A-2012-138306.
According to JP-A-2012-138306, the content of Al in the first metal component is adjusted to preferably 99.5 wt % or more in order to obtain excellent electrical conductivity besides high joint strength. Such a first metal component only includes 0.5 wt % or less of inevitable impurities, and can realize an electric conductivity of 60% IACS (an international standard for electric resistance) or even higher. As examples of such a first metal component, aluminum alloys 1050, 1080, 1100 (JIS), etc. can be mentioned. Furthermore, the content of Cu in the second metal component is preferably adjusted to 99.9 wt % or more. Such a second metal component only includes 0.1 wt % or less of inevitable impurities, and can realize an electric conductivity of about 90% IACS. As an example of such a second metal component, oxygen-free copper can be mentioned.
The joint part includes at least one “joint element” selected from the group consisting of Si, Ni, Mn, Co, Zn, Ge, Au, Ag and Pd. In the joint material having such a composition, reactivity between Al and the at least one joint element selected therefrom is higher than reactivity between Al and Cu. Additionally, reactivity between Cu and at least one joint element selected therefrom is higher than reactivity between Cu and Al. That is, Al and Cu react more preferentially with the at least one joint element, rather than reacting with each other, in cases where Al, Cu, and the at least one joint element coexist.
Accordingly, by employing such a joint part including the joint element(s), the first metal component and the second metal component can rigidly be joined through the joint part while a reaction between Al and Cu is suppressed. The conventional art describes that, as a result, a highly-reliable joint material can be obtained.
Furthermore, a method for welding thin heterogenous-metal plates, including: irradiating a side of a first thin metal plate having a relatively-low melting point, with an energy beam, to form spot-like welded parts, has been disclosed in WO2006/016441.
According to WO2006/016441, a cross-section of one welded part is wedge-shaped so as to taper toward a direction from the upper positive-electrode terminal to the lower negative-electrode terminal. Within the welded part, at least a penetration part and an adjacent part thereof are made of an alloy. Within the stacked area, the total area (total amount) of the welded part itself is small. Therefore, any increase in the electric resistance due to the alloyed welded part is reduced, and thus, the stacked area exhibits favorable electric properties.
Furthermore, intermediate regions in which positive terminals and negative terminals are in direct contact with each other are formed between adjacent welded parts, and therefore, an electric current can be caused to flow through the intermediate regions having reduced electric resistance. Accordingly, preferable electric properties can be retained. In the intermediate regions, the terminals on the both upper and lower sides are joined with each other via welded parts, and therefore, the positive-electrode terminals and the negative-electrode terminals come into sufficiently close contact with each other. Accordingly, any gaps are hardly generated between the terminals, and thus, electric currents flow therethrough in a favorable manner.
Although WO2006/016441 does not discuss joint strength, an increase in the electric resistance and a decrease in the joint strength are caused from production of large amounts of intermetallic compounds. From the perspective of suppression of such production of intermetallic compounds, the direction of WO2006/016441 is the same as the direction of JP-A-2012-138306.